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Mystery Solved on Less Methane in Air: Leakage Control

Published: August 22nd, 2012

Carbon dioxide grabs all the headlines, but methane is also a greenhouse gas, and because it traps heat a lot more efficiently than CO2 does, it’s drawing increasing attention from climate warriors. What’s slightly baffling, however, is the fact that the growth of methane concentrations in the atmosphere has actually slowed over the past few decades, even as methane-generating activities — drilling for fossil fuels, for example, and agriculture — have expanded.

Now a team of atmospheric chemists, writing a report in Nature that was released Wednesday, have solved that riddle: there’s a lot less methane leaking from oil and gas wells than there used to be. Methane is a major component of natural gas, and, said lead author Isobel Simpson, of the University of California-Irvine, in an interview, “as natural gas has become more valuable, it’s being captured rather than vented or flared.”

The answer is pretty straightforward, but obtaining it wasn’t. The problem is that methane comes from a wide range of sources, both natural and artificial. On the artificial side, it leaks from drilling rigs, belches from the stomachs and ferments from the manure of cows, and bubbles up from rice paddies. It also comes purely naturally from swamps and natural deposits under the sea floor and from the guts of termites. With so many sources, it’s hard to figure out which one is responsible. “One study last year said that emissions from rice paddies have been decreasing,” Simpson said, “but a later study said the opposite.”

So rather than looking at methane concentrations, Simpson and her colleagues looked at ethane, a non-greenhouse gas that has many fewer sources, and just one — leakage from fossil-fuel exploration — that it shares with methane. The team used a global gas-monitoring network Irvine scientists set up in the 1970s to determine that ethane emissions dropped by a whopping 21 percent from 1984 to 2010.

They were able to rule out biomass and biofuel burning as the explanation for the drop, leaving only leakage control. And since controlling leaks would reduce methane emissions just as much as it would ethane, it’s clear that the same explanation applies to the slower buildup in atmospheric methane. “We modeled it out,” Simpson said, “and confirmed that the major sources are these fugitive emissions.”

That might seem like favorable news for the climate, but the good times may be ending. “The growth rate in methane concentrations,” Simpson said, “had leveled off, but it now appears to be growing again.” It’s possible that the boom in natural-gas fracking could be the cause. “We’re not seeing that signal in our network,” Simpson said, “but we haven’t looked in detail at the latitudes where fracking has expanded.”

Without good evidence, Simpson and her co-authors won’t speculate about what’s going on. But even if emissions from drilling and agriculture remain under control, climate scientists worry about another potential source of heat-trapping methane. As the Arctic warms, those gas deposits trapped under the sea floor could escape in large amounts, turbocharging the warming already under way.

All of which means that whether or not we can control our own methane emissions, nature could have some nasty surprises of its own.